How to paint a texture on, based on the way it was mapped on with UV?
In texture paint mode you can either 


Paint and modify the existing texture image such as (.png) file.
Paint on a new texture and layer the textures so that one covers the others.
Paint with a brush from one image to another final image ... rather like a clone tool from GIMP


A video tutorial for texture painting is probably a necessary resource for ease of explanation and understanding.